Increasing the pixel resolution of small RGB displays causes a severe loss in aperture and consequently brightness. The implementation of a multi-primary sub-pixel layout of the pixels of the display together with sub-pixel rendering allows the use of larger sub-pixels and increased transmission through the color filters, and hence an increased brightness without much influence on the perceived resolution. A reduction of the pixel resolution by using a multi-primary display and application of sub-pixel rendering enables to use less drivers.
For full color reproduction, a multi-primary display is a display with more than the three standard primaries, which usually are red R, green G, and blue B. An example of a multi-primary display is an RGBW display of which the pixels comprise R, G, B and white W sub-pixels. In such an RGBW display, the transmission of light through the pixel is greatly increased because no color filter is required for the W sub-pixel. However, the gamut is reduced because this W sub-pixel can not be activated for high brightness saturated colors. A second advantage is the increased resolution through sub-pixel rendering.
Some examples of known sub-pixel configurations of RGBW displays are the quad pixel configuration, the pentile configuration and the vertical stripe configuration. Examples of other existing multi-primary displays are RGBY displays wherein one of the sub-pixels is yellow Y, or RGBCY displays in which the pixels comprise additional cyan C and yellow Y sub-pixels.
The basic reason why sub-pixel rendering increases the resolution is that each sub-pixel is able to convey luminance information at a higher resolution than the full pixel. The effectiveness of sub-pixel rendering for a particular sub-pixel configuration is strongly influenced by how many luminance points can be assigned to each pixel, and how strong these luminance points are. With strong is meant the maximum luminance reachable and having a more similar color. In an RGBW display the two luminance points W and RGB are very strong, both the first group of sub-pixels which comprises the W sub-pixel and the second group of sub-pixels which comprises the R, G, and B sub-pixels are able to produce the same white light with a high intensity. Further, the luminance of the W sub-pixel may be very high.
A state of the art video chain for sub-pixel rendering may comprise a scaling unit, a pre-filter, a multi-primary conversion and a sub-pixel mapping. The scaling unit receives an RGB image with arbitrary resolution and supplies an RGB image at full resolution matching the luminance points resolution of the display. Or said differently, in the full resolution RGB image an RGB sample exists for each sub-pixel of the display. The image may be a still image or video, and may comprise synthetic and/or natural information. The synthetic information may be computer generated information such as, for example, text and/or graphs. The natural information may be, for example, a photograph or film. Preferably, the input image has image detail that corresponds to what can be represented by the luminance points of the display. The pre-filter filters the RGB full resolution image to remove (chroma) detail which cannot be represented by the sub-pixel rendering without visible artifacts. Thus, detail is lost, but color and luminance are maintained. The multi-primary converter converts the filtered RGB signal into a full resolution RGBW signal. Or, more general, converts the three primary input signal into the multi-primary signals associated with the more than three sub-pixels per pixel of the display. The sub-pixel mapper generates the drive values for the sub-pixels by selecting them from the full resolution RGBW signal depending on the primary dictated by the sub-pixel pattern for the location of the sub-pixel. However, such existing sub-pixel rendering algorithm has the drawback that the readability of text, and the representation of fine details and datagraphic images is poor.